Sheet feed roller

ABSTRACT

A sheet feed roller capable of achieving highly accurate sheet feed distance has an outer peripheral surface including a feed surface that extends at least locally in an axial direction of the roller over an entire circumference of the roller. The feed surface of the roller is provided with a plurality of projections, including microscopic spikes that can be pierced into the sheet, and stoppers for limiting a piercing depth of the spikes in the sheet.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an improved sheet feed roller for feeding various types of sheets in imaging machines, such as printers, copying machines or facsimile machines, so as to print information onto the sheet or read out information from the sheet.

[0003] 2. Description of Related Art

[0004] Sheet feed rollers are generally classified into two types. A first type makes use of rollers having an outer peripheral surface with a high friction coefficient. In this instance, the sheet is sandwiched between the feed roller and a pinch roller and transferred primarily by friction force. The feeding of the sheet relies upon unstable friction force, and it is often difficult to achieve a sufficient feeding accuracy. For overcoming such difficulty associated with the feed rollers of the first type and improving the feeding accuracy of the sheet, there has been proposed a second type wherein the outer peripheral surface of the roller is provided with a plurality of microscopic spikes that can be pierced into the sheet. In this instance, the sheet is positively transferred under engagement of the spikes and corresponding microscopic recesses formed in the sheet by the spikes. The latter type of sheet feed rollers are disclosed, for example, in JP 08-310703A, JP 10-109777A, JP 10-203675A, JP 10-236683A, JP 2000-159377A, JP 2000-159378A and JP 2000-159379A.

[0005] However, it has been found by the inventors that even the sheet feed rollers with the microscopic spikes may not realize a satisfactory feeding accuracy, depending upon the material of the sheet to be transferred, or the pressure under which the spikes are pierced into the sheet. Here, the feeding accuracy is typically represented by the difference between the desired feeding distance and the actual feeding distance, per unit rotation of the feed roller.

[0006] The inventors conducted thorough research and investigations to seek measures for improving the feeding accuracy of the feed rollers, and found the mechanism whereby unsatisfactory feeding accuracy occurs, as follows. That is to say, the feeding accuracy of the feed roller provided with the microscopic spikes is degraded by fluctuation of the piercing depth of the spikes, which occurs depending upon the material of the sheet to be fed and/or the pressure for urging the feed roller against the sheet. When the piercing depth of the spike is insufficient, there occurs fluctuation of the sheet feeding between the spike and the recess formed in the sheet by the spike. Fluctuation of the piercing depth of the spike also causes fluctuation of the sheet feeding. On the other hand, when a required piercing depth of the spike is achieved by a sufficient urging force of the roller against the sheet, in an attempt to avoid occurrence of fluctuation of the sheet feeding, the sheet feeding radius changes depending upon the piercing depth and inevitably causes fluctuation of the sheet feeding. The above-mentioned mechanism will be more fully explained below with reference to FIGS. 1(a), 1(b) and 1(c) and FIG. 2.

[0007]FIG. 1(a) is a sectional view of a sheet feed roller 10, wherein the projection 11 is in the form of a spike 12 having a height H, and the spike 12 is pierced into the sheet 21A under an urging force F₁. It is assumed that the sheet 21A is relatively hard, and the piercing depth of the spike 12 into the sheet 21A is D₁ and the feeding radius of the sheet 21A is R₁. In this instance, the piercing depth D₁ of the spike 12 is insufficient so that the spike 12 moves as shown by imaginary line 12′, without being synchronized with the recess 22 in the sheet 21A, thereby causing fluctuation in sheet feeding. The actual feeding distance of the feed roller 10 deviates from the desired feeding distance and reduced by an amount corresponding to the fluctuation of the sheet feeding.

[0008] When the urging force F₁ is increased so as to increase the piercing depth D₁ from the state shown in FIG. 1(a), it is possible to decrease fluctuation in sheet feeding. As shown in FIG. 1(b), an optimum piercing depth D₀ is achieved under an increased urging force F₀, with which the fluctuation in sheet feeding is decreased to a negligible level. In this instance, the sheet feeding radius R₀ is a predetermined, optimum value and the slipping rate between the feed roller and the sheet is substantially zero so that a predetermined sheet feeding distance 2πR₀ is achieved for each rotation of the feed roller.

[0009] In this way, it is possible to achieve a predetermined feeding distance 2πR₀ under an increased urging force F₀, insofar as a relatively hard sheet 21A is concerned. When, however, a relatively soft sheet 21B is to be fed by the feed roller under the same urging force F₀, there arises a tendency that the predetermined sheet feeding distance 2πR₀ is not achieved. Thus, as shown in FIG. 1(c), when the spike 12 of the feed roller 10 is pierced into the relatively soft sheet 21B, the piercing depth D₂ is larger than the optimum depth D₀ since the sheet 21B exhibits a relatively small piercing resistance. In this instance, because the distance between the center axis of the feed roller and the tip end of the spike 12 is not changed, the sheet feeding radius R₂ is smaller than the predetermined value R₀. Therefore, when a relatively soft sheet 21B is to be fed under an increased urging force F₀ that is made optimum for feeding a relatively hard sheet 21A without noticeable fluctuation in sheet feeding, the sheet feeding distance per unit rotation of the feed roll is decreased to 2πR₂ that is smaller than the predetermined distance 2πR₀.

[0010] In order to eliminate the above-mentioned problems, it is necessary to achieve a constant piercing depth D₀ irrespective of the hardness of the sheet. To this end, there may be used a feed roller 30 as shown in FIG. 2, wherein microscopic projections in the form of spikes 32 having a triangular section are provided on the outer surface 30A of the roller 30. In this instance, as with the case of the spikes 12 shown in FIG. 1(b), the spikes 32 under the same urging force F₀ are not only pierced into a relatively hard sheet 21A with the desired piercing depth D₀, but also pierced into a relatively soft sheet 21B with a piercing depth that is not increased beyond the desired depth D₀, due to a contact of the lower surface of the sheet 21B with the outer surface 30A of the roller 30.

[0011] However, the feed roller 30 of the type shown in FIG. 2 is not easy to produce efficiently and at reasonable cost, since it would be necessary either to subject the outer surface 30A of the roller 30 to grinding or the like machining so as to remove materials and thereby leave the microscopic spikes 32 on the outer surface 30A, or to join separately prepared microscopic spikes 32 to the flat outer surface 30A of the roller 30.

[0012] An alternative method for producing the feed roller 30 of the type shown in FIG. 2 is disclosed in the patent documents cited above, wherein a round rod is subjected to roll forming so that the material at the outer surface of the rod is raised to form the spikes. While such a method makes it possible to produce the feed roller 30 efficiently and at low cost, there arises a problem that even when it is desired to form a microscopic spike 32 having exactly triangular section and height D₀, limitations imposed on the production technology make it inevitable that a spike 32 having a somewhat flared root portion is formed. As a result, it is still impossible to maintain substantially constant the piercing depth of the spike 32 as it is pierced into a relatively soft sheet, since the piercing depth depends on the hardness of the sheet and the piercing resistance of the spike at its flared root portion. Therefore, the problem of fluctuation in sheet feeding radius or sheet feeding distance remains unsolved.

SUMMARY OF THE INVENTION

[0013] It is therefore an object of the present invention to eliminate the problems of the prior art mentioned above, and to provide an improved sheet feed roller capable of achieving a highly precise sheet feeding distance without fluctuations, irrespective of the hardness of the sheet, and suitable for production at high manufacturing productivity and at low cost.

[0014] To this end, according to the present invention, there is provided a sheet feed roller having an outer peripheral surface, said outer peripheral surface including at least one feed surface region that extends at least locally in an axial direction of the roller and over an entire circumference of the roller, said feed surface being provided with a plurality of projections, said plurality of projections being comprised of microscopic spikes that can be pierced into the sheet, and stoppers for limiting a piercing depth of the spikes in the sheet.

[0015] With the sheet feed roller according to the present invention, since the projections on the feed surface of the feed roller are comprised of microscopic spikes that can be pierced into the sheet, and stoppers for limiting a piercing depth of the spikes in the sheet, it is always possible to maintain the optimum piercing depth of the spikes by the stoppers even when the hardness of the sheet changes from time to time. In this way, the desired sheet feeding radius or distance can be maintained without causing fluctuations, thereby realizing a highly precise sheet feeding.

[0016] In the case of a sheet feed roller for ink jet printers, for example, due to limitations in machine design, the total urging force applied to the sheet feed roller in use is made relatively low. In order to achieve an optimum piercing depth for each spike, it is sometimes necessary to reduce the number of the spikes that are simultaneously in engagement with the sheet. Thus, it is preferred that each of the projections comprises one of the spike and the stopper. In this instance, it is possible to reduce the number of the spikes that are simultaneously in engagement with the sheet since projection comprising the stoppers may be arranged adjacent to the projections comprising the spikes, and the distance between the adjacent spikes can be increased. Such an arrangement may also be advantageous when the distance between the adjacent projections cannot be readily reduced due to roll forming conditions or the like.

[0017] Alternatively, each of the projections comprising the spikes may further comprise the stopper. In this instance, it is possible to ensure that each spike can be pierced into the sheet by a constant, optimum piercing depth since the stopper of the projection limit the radial position of the sheet relative to the feed roller.

[0018] It is preferred that each of the projections is arranged in that region of the feed surface, which is defined by first helices extending in parallel with each other on the outer peripheral surface, and second helices extending in parallel with each other on the outer peripheral surface, wherein the second helices are crossed with the first helices. Here, the term “helices” signifies helical lines that extend in the axial direction of a cylindrical body, along the outer peripheral surface thereof. In this instance, it is possible to efficiently form the projections by a roll forming device comprising a first roll forming die for forming grooves on the outer surface of the feed roller so as to extend along the first helices, and a second roll forming die for forming grooves on the outer surface of the feed roller so as to extend along the second helices, thereby minimizing the production cost of the sheet feed roller.

[0019] In the arrangement described above, at least one projection comprising the spike may be arranged alternately with at least one projection comprising the stopper, along the first or second helices. Such an arrangement of the spikes and the stoppers makes it readily possible to ensure that each spike can be pierced into the sheet by a constant, optimum-piercing depth.

[0020] It is alternatively preferred that each of the projections is arranged in that region of the feed surface, which is defined by generatrices extending along the outer peripheral surface in parallel with an axial direction of the roller, and circumferential lines extending in parallel with each other on the outer peripheral surface, wherein the circumferential lines are crossed with the generatrices. Here, the term “generatrices” signifies straight lines that, in the case of a cylindrical body, extend axially along the outer peripheral surface of the cylindrical body. In this instance also, it is possible to efficiently form the projections by a roll forming device comprising a first roll forming die for forming grooves on the outer surface of the feed roller so as to extend along the circumferential lines, and a second roll forming die for forming grooves on the outer surface of the feed roller so as to extend along the circumferential lines, thereby minimizing the production cost of the sheet feed roller.

[0021] In the arrangement described above, at least one projection comprising the spike may be arranged alternately with at least one projection comprising the stopper, along the generatrices or the circumferential lines. Such an arrangement of the spikes and the stoppers makes it readily possible to ensure that each spike can be pierced into the sheet by a constant, optimum piercing depth.

[0022] It is further preferred that the stopper has a flat surface that is substantially at right angles to a radial direction of the roller. The flat surface of the stopper serves to positively maintain the desired optimum piercing depth of the spikes, in highly accurate manner

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The present invention will be more fully explained below with reference to some preferred embodiment shown in the accompanying drawings.

[0024] FIGS. 1(a) to 1(c) are sectional views showing a conventional sheet feed roller.

[0025]FIG. 2 is a sectional view showing another conventional sheet feed roller.

[0026]FIG. 3(a) is a perspective view showing a sheet feed roller according to a first embodiment of the present invention, and FIG. 3(b) is a perspective view showing a modification thereof.

[0027]FIG. 4 is a developed view of the sheet feeding surface region of the feed roller shown in FIG. 3(a) or 3(b).

[0028]FIG. 5 is a sectional view corresponding to section 5-5 in FIG. 4, but showing the feed roller in use.

[0029]FIG. 6 is a view showing the arrangement of a roll forming device for forming the feed roller according to the first embodiment.

[0030]FIG. 7 is a perspective view showing a first die of the roll-forming device shown in FIG. 6.

[0031]FIG. 8 is a perspective view showing a second die of the roll-forming device shown in FIG. 6.

[0032]FIG. 9 is a perspective view showing a sheet feed roller according to a second embodiment of the present invention.

[0033]FIG. 10 is a developed view of the sheet feeding surface region of the feed roller shown in FIG. 9.

[0034]FIG. 11 is a view showing the arrangement of a roll forming device for forming the feed roller according to the second embodiment.

[0035]FIG. 12 is a perspective view showing a first die of the roll-forming device shown in FIG. 11.

[0036]FIG. 13 is a perspective view showing a second die of the roll-forming device shown in FIG. 11.

[0037]FIG. 14 is a perspective view showing a sheet feed roller according to a third embodiment of the present invention.

[0038]FIG. 15 is a developed view of the sheet feeding surface region of the feed roller shown in FIG. 14.

[0039]FIG. 16 is a sectional view corresponding to section 16-16 in FIG. 15, but showing the feed roller in use.

[0040]FIG. 17 is a view showing the arrangement of a roll forming device for forming the feed roller according to the third embodiment.

[0041]FIG. 18 is a perspective view showing a die of the roll-forming device shown in FIG. 17.

[0042]FIG. 19 is a graph showing the deviation of sheet feeding distances obtained by performance tests under various surface pressure conditions.

[0043]FIG. 20 is a developed view similar to FIG. 4, showing the sheet feeding surface region of the modified feed rollel

[0044]FIG. 21 is a graph showing the deviation of sheet feeding distances obtained by further performance tests under various surface pressure conditions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] A first embodiment of the present invention is shown in FIG. 3(a), wherein the sheet feed roller is generally denoted by reference numeral 101. The feed roller 101 may be suitably used in an imaging machine, such as printers, copying machines or facsimile machines, for feeding a sheet S on which image information is printed. The feed roller 101 includes a cylindrical roller body 102 provided on both axial ends with shaft end portions 103 for rotatably supporting the feed roller 101 in the imaging machine. The feed roller 101 has an outer peripheral surface provided with at least one feed surface region 104 that extends at least locally in an axial direction of the roller 101 and over an entire circumference thereof. In the embodiment shown in FIG. 3(a), there are provided three feed surface regions 104 that are spaced from each other in the longitudinal direction of the feed roller 101.

[0046] A modification is shown in FIG. 3(b), wherein the sheet feed roller is generally denoted by reference numeral 101A includes a cylindrical roller body 102A provided on both axial ends with shaft end portions 103A for rotatably supporting the feed roller 101A in the imaging machine. The feed roller 101A differs from that shown in FIG. 3(b) essentially in that the outer surface of the roller 101A as a whole constitutes a feed surface region 104A.

[0047] The sheet feed rollers 101, 101A are rotatably mounted in the imaging machine with their feed surface regions 104, 104A in pressure contact with pinch rollers PR so that the sheet S sandwiched between the feed surface regions 104, 104A and the pinch rollers PR is highly accurately fed toward the downstream side of the feed roller 101, 101A.

[0048] As mentioned above, FIG. 4 is a developed view of the sheet feeding surface region 104, 104A of the feed roller 101, 101A shown in FIG. 3(a) or 3(b), and FIG. 5 is a sectional view corresponding to section 5-5 in FIG. 4, but showing the feed roller 101, 101A in use. It can be seen that the entire feed surface region 104 of the roller 101 is comprised of a number of diamond-shaped microscopic areas defined by a plurality of first helices L₁ that are in parallel with each other, and a plurality of second helices L₂ that are also in parallel with each other but arranged so that they are crossed with the first helices L₁. Each of such microscopic areas is provided with a microscopic first projection 105 or a microscopic second projection 106, which are combined with each other such that the first and second projections 105 and 106 are arranged alternately with each other along the first helices L₁, and either the same first projections 105 or the same second projections 106 are arranged continuously along the second helices L₂.

[0049] In FIG. 4, reference character R denotes a direction parallel to the circumferential direction of the sheet feeding region 104, and reference character W denotes a direction parallel to the center axis of the roller 101. The first helices L₁ are oriented so as to form an angle of 45° with reference to the axial direction W of the roller 101 and spaced from each other by a pitch P₁. The second helices L₂ are oriented so as to form an angle of −45° with reference to the axial direction W of the roller 101 and spaced from each other by the same pitch P₁. Thus, as shown in FIG. 5, the projections 105 and 106 are arranged alternately with each other in the axial direction W of the roller 101 so as to be spaced from each other by a pitch P₂. It is to be noted, however, that the directions and the pitches of the first and second helices L₁, L₂ are not limited to those of the embodiments shown in FIG. 4.

[0050] The first projection 105 is in the form of a microscopic pyramid having a height H₁. The first projection 105 has a spike 105A at it top portion, which can be pierced into the sheet S so as to feed the sheet, and four facets 105B, 105C of which two facets 105B are opposed to the first helices L₁ and the other two facets 105C are opposed to the second helices L₂. The first projection 105 forms an angle θ between opposite edges of each facet 105B, 105C. The second projection 106 is in the form of a frustum of a microscopic pyramid having a height H₂ that is lower than the height H₁ of the first projection 105. The second projection 106 has a flat top surface 106A, which can be brought into engagement with the surface of the sheet S as a stopper for limiting the piercing depth D of the spike 105A of the adjacent first projection 105, and four facets 106B, 106C, of which two facets 106B are opposed to the first helices L₁ and the other two facets 106C are opposed to the second helices L₂. The second projection 106 forms an angle θ between opposite edges of each facet 106B, 106C.

[0051] It is preferred that the opposite facets 105B or 105C of the first projection 105 form an angle φ that is within a range of 30° to 60°. It is to be noted that the angle φ between the opposite facets 105B or 105C of the first projection 105 is slightly different from the angle θ between the opposite edges of each facet 105B, 105C. If the angle φ between the opposite facets 105B or 105C of the first projection 105 is larger than 60°, there may be instances wherein a sufficient piercing depth D for a relatively hard sheet cannot be achieved, thereby causing slipping of the sheet while it is being fed. On the other hand, if the angle φ is smaller than 30°, there may be instances wherein the mechanical strength of the spike 105A is insufficient, thereby degrading the durability of the feed roller.

[0052] It is also preferred that the piercing depth D of the first projection 105 is within a range of 10 μm to 40 μm. If the piercing depth D is smaller than 10 μm, there may be instances wherein slipping of the sheet occurs due to insufficient piercing depth. On the other hand, if the piercing depth D is larger than 40 μm, there may be instances wherein the surface of the sheet S cannot be properly supported by the stoppers 106A of adjacent second projections 106 particularly when the sheet S is relatively hard, thereby causing fluctuation in the sheet feeding radius depending upon the hardness of the sheet S.

[0053] As for the first projection 105 having a spike 105A to be pierced into the sheet S, although the tip of the spike 105A may be sharp from the viewpoint of piercing function, it is often preferred from the viewpoint of manufacturing technology that the spike 105A is in the form of a frustum of a pyramid. In this instance, it is preferred that the spike has a top surface with a surface area not greater than 400 μm², more preferably not greater than 100 μm², and more preferably not greater than 50 μm². As for the second projection 106, while it is desirable for the top surface 106A to have as large a surface area as possible, from the viewpoint of the stopper function, it is often preferred from the viewpoint of manufacturing technology that the tip surface 106A has a surface area within a range of 160-3600 μm², more preferably 400-2500 μm². Furthermore, although the first projection 105 in the illustrate embodiment is in the form of a pyramid, it may be in the form of a cone provided that it has a spike that can be pierced into the sheet S. Also, although the second projection 106 in the illustrate embodiment is in the form of a frustum of a pyramid, it may be in the form of a frustum of a cone, provided that it has a top surface serving as a stopper for limiting the piercing depth D of the first projection 105.

[0054] When the roller body 102 is comprised of a metal material, the sheet feeding surface region 104 can be advantageously formed by a pair of roll forming dies by a roll forming process to be described hereinafter. FIG. 6 shows the arrangement of a roll forming device comprising a first die 120 and a second die 121 for forming the feed roller according to the embodiment of FIG. 3(a), and FIGS. 7 and 8 are perspective views showing the first die 120 and the second die 121, respectively.

[0055] The first roll forming die 120 has an outer surface provided with ridges 120A that are arranged at a constant distance over the entire periphery thereof. Neighboring ridges 120A are spaced from each other with a groove 120B therebetween, wherein the groove 120B is of triangular cross-section. Each ridge 120A is of a trapezoidal cross-section, and has a cutting surface 120C on its top, for forming grooves along the first helices L₁ on the sheet feeding surface region 104. Similarly, the second roll forming die 121 has an outer surface provided with ridges 121A that are arranged at a constant distance over the entire periphery thereof. Neighboring ridges 121A are spaced from each other alternately with a groove 121B and another groove 121C therebetween, wherein the groove 121B is of triangular cross-section and the groove 120C is of trapezoidal cross-section. Thus, for example, a groove 121B with triangular cross-section is arranged between the first and the second ridges 121A, 121A, and a groove 121C with trapezoidal cross-section is arranged between the second and the third ridges 121A, 121A, and such an arrangement of the ridges and the grooves is repeated in the circumferential direction of the second roll forming die. Here also, each ridge 121A of the second die 121 is substantially of trapezoidal cross-section, and has a cutting surface 121D on its top, for forming grooves along the second helices L₂ on the sheet feeding surface region 104.

[0056] When the sheet feeding surface regions 104 are to be formed on a roller body 102, the roller body 102 is clamped between the first and second roll forming dies 120, 121, which are arranged with their respective center axes in parallel with each other. By urging the roller body 102 against the first and second roll forming dies 120, 121 under a predetermined working pressure, and rotating these dies 120, 121, it is possible to form the desired feed surface region 104 on the roller body 102.

[0057] As seen in exploded views, the angle formed between the ridge 120A and the center axis of the first roll forming die 120 is the same as the angle (e.g., +45°) between the first helices L₁ on the sheet feeding surface region 104 and the center axis of the feed roller 101. Similarly, the angle formed between the ridge 121A and the center axis of the second roll forming die 121 is the same as the angle (e.g., −45°) between the second helices L₂ on the sheet feeding surface region 104 and the center axis of the feed roller 101.

[0058] With such an arrangement of the roll forming device, the facets 105B, 106B of the projections 105, 106 opposed to the first helices L₁ are formed by the wall surfaces 120D of the triangular grooves 120B in the first die 120, the facets 105C of the projections 105 opposed to the second helices L2 are formed by the wall surfaces 121E of the triangular grooves 121B in the second die 121, the facets 106C opposed to the second helices L2 are formed by the wall surfaces 121F of the trapezoidal grooves 121C in the second die 121, and the stoppers 106A of the second projections 106 are formed by the bottom surfaces 121G of the trapezoidal grooves 121C of the second die 121.

[0059] In the roll forming device shown in FIGS. 6 to 8, all of the grooves in the first roll forming die 120 are comprised of triangular grooves 120B. It is however possible to arrange one or more trapezoidal grooves between neighboring triangular grooves 120B. As for the second roll forming die 121, it is likewise possible to arrange two or more trapezoidal grooves 121C between neighboring triangular grooves 121B. By appropriately selecting the number of the trapezoidal grooves provided for the roll forming dies 120, 121, it is possible to realize a desired arrangement of the microscopic projections 105, 106 on the sheet feeding surface region 104 wherein the number of the stoppers 106A is optimized for each spike 105A.

[0060] As mentioned above, the first and second dies 120, 121 of the roll forming device shown in FIGS. 6-8 are arranged with their respective center axes in parallel with each other, as mentioned above. The roller body 102 is oriented in parallel with the dies 120, 121 and urged against the dies 120, 121 under a predetermined working pressure, while the dies 120, 121 are rotated. In this instance, it is possible to form the sheet feeding surface region 104 on the roller body 102 without causing an axial movement of the roller body 102 relative to the first and second dies 120, 121, provided that the width of the sheet feeding surface region 104 on the roller body 102 as seen in the axial direction is the same as the width of the dies 120, 121. This type of roll forming method is known as infeed roll forming process.

[0061] When such an infeed roll forming process is applied to formation of the sheet feeding surface region 104A of the sheet feed roller 101A shown in FIG. 3(b), which extends over the entire length of the roller body 102A, the roll forming dies 120, 121 must have a large width corresponding to the axial length of the sheet feeding surface region 104A, thereby making it difficult to achieve an uniform roll forming over the entire length of the roller body 102A. In order to eliminate such difficulty, it is preferred to carry out a thru-feed roll forming process wherein the roll forming device has a slightly different arrangement in that the center axis of the first die 120 is inclined relative to the center axis of the roller body 102B by a predetermined angle, and the center axis of the second die 121 is oppositely inclined relative to the center axis of the roller body 102B by the same angle of the opposite sign, without causing intersection of the center axes of the dies 120, 121 with the center axis of the roller body 102B. In this instance, the roller body 102 is urged against the dies 120, 121 under a predetermined working pressure, while the first and second roll forming dies 120, 121 are rotated, so as to feed the roller body 102 axially relative to the dies 120, 121 under a predetermined speed, and thereby form the feed surface region 104 uniformly over the roller body 102.

[0062] A second embodiment of the present invention is shown in FIG. 9, wherein the sheet feed roller is generally denoted by reference numeral 131. The feed roller 131 includes a cylindrical roller body 132 provided on both axial ends with shaft end portions 133 for rotatably supporting the feed roller 131 in the imaging machine. The feed roller 131 has an outer peripheral surface provided with three sheet feeding surface regions 134 that are spaced from each other axially and extend over an entire circumference of the roller 131.

[0063] The sheet feed roller 131 is rotatably mounted in the imaging machine with its sheet feeding surface regions 134 in pressure contact with pinch rollers PR so that the sheet S sandwiched between the feed surface regions 134 and the pinch rollers PR is highly accurately fed toward the downstream side of the feed roller 131.

[0064] As mentioned above, FIG. 10 is a developed view of the sheet feeding surface region 134 of the feed roller 131 shown in FIG. 9, wherein reference character R denotes a direction parallel to the circumferential direction of the sheet feed roller 131, and reference character W denotes a direction parallel to the center axis of the feed roller 131. It can be seen that each feed surface region 134 of the roller 131 is comprised of a number of rectangular microscopic areas defined by a plurality of circumferential lines L₃ that extend over the sheet feeding surface region 134, and a plurality of generatrices L4 that extends axially over the sheet feeding surface region 134. Each of such microscopic areas is provided with a microscopic first projection 135 or a microscopic second projection 136, which are combined with each other such that the first and second projections 135 and 136 are arranged alternately with each other along the generatrices L₄, and either the same first projections 135 or the same second projections 136 are arranged continuously along the circumferential lines L₃.

[0065] The first projection 135 is in the form of a microscopic pyramid having a spike 135A at it top portion, which can be pierced into the sheet S so as to feed the sheet, and four facets 135B, 135C of which two facets 135B are opposed to the circumferential lines L₃ and the other two facets 135C are opposed to the generatrices L₄. The second projection 136 is in the form of a frustum of a microscopic pyramid having a flat top surface 136A, which can be brought into engagement with the surface of the sheet S as a stopper for limiting the piercing depth D of the spike 135A of the adjacent first projection 135, and four facets 136B, 136C, of which two facets 136B are opposed to the circumferential lines L₃ and the other two facets 136C are opposed to the generatrices L₄.

[0066] When the roller body 132 is comprised of a metal material, the sheet feeding surface region 134 can be advantageously formed by a pair of roll forming dies by a roll forming process to be described hereinafter. FIG. 11 shows the arrangement of a roll forming device comprising a first die 140 and a second die 141 for forming the feed roller according to the embodiment of FIG. 9, and FIGS. 12 and 13 are perspective views showing the first die 140 and the second die 141, respectively.

[0067] The first roll forming die 140 has an outer surface provided with ridges 140A that are arranged at a constant distance over the entire periphery thereof. Each ridge 140A has a flat top surface 140D for forming grooves in the sheet feeding surface region 134 so as to extend along the. Neighboring ridges 140A are spaced from each other alternately with a groove 140B or another groove 140C therebetween, wherein the groove 140B is of triangular cross-section and the groove 140C is of trapezoidal cross-section. Each ridge 140A is of trapezoidal cross-section, and has a cutting surface 140D on its top, for forming grooves along the circumferential lines L₃ on the sheet feeding surface region 134. Similarly, the second roll forming die 141 has an outer surface provided with ridges 141A that are arranged at a constant distance over the entire periphery thereof. Neighboring ridges 141A are spaced from each other alternately with a groove 141B, which is of triangular cross-section. Here also, each ridge 141A of the second die 141 is substantially of trapezoidal cross-section, and has a cutting surface 141C on its top, for forming grooves along the generatrices L₄ on the sheet feeding surface region 134.

[0068] With such an arrangement of the roll forming device, the facets 135C, 136C of the projections 135, 136 opposed to the generatrices L4 are formed by the wall surfaces 141D of the triangular grooves 141B of the second die 141, the facet 135B of the projection 135 opposed to the circumferential lines L₃ are formed by the wall surfaces 140E of the triangular grooves 140B of the first die 140, the facets 136B of the projection 136 opposed to the circumferential lines L3 are formed by the wall surfaces 140F of the trapezoidal grooves 140C of the first die 140, and the stoppers 136A of the projection 136 are formed by the bottom surfaces 140G of the trapezoidal grooves 140C of the first die 140.

[0069] In the roll forming device shown in FIGS. 10-12, it is possible to arrange two or more trapezoidal grooves 140C between neighboring triangular grooves 140B. By appropriately selecting the number of the trapezoidal grooves provided for the roll forming dies 140, 141, it is possible to realize a desired arrangement of the microscopic projections 135, 136 on the sheet feeding surface region 134 wherein the number of the stoppers 136A is optimized for each spike 135A.

[0070] A third embodiment of the present invention is shown in FIG. 14, wherein the sheet feed roller is generally denoted by reference numeral 151. The feed roller 151 includes a cylindrical roller body 152 provided on both axial ends with shaft end portions 153 for rotatably supporting the feed roller 151 in the imaging machine. The feed roller 151 has an outer peripheral surface provided with three sheet feeding surface regions 134 that are spaced from each other axially and extend over an entire circumference of the roller 151.

[0071] The sheet feed roller 151 is rotatably mounted in the imaging machine with its sheet feeding surface regions 154 in pressure contact with pinch rollers PR so that the sheet S sandwiched between the feed surface regions 154 and the pinch rollers PR is highly accurately fed toward the downstream side of the feed roller 151.

[0072] As mentioned above, FIG. 15 is a developed view of the sheet feeding surface region 154 of the feed roller 151 shown in FIG. 14, and FIG. 16 is a sectional view corresponding to section 16-16 in FIG. 15, but showing the feed roller 151 in use. It can be seen that the entire feed surface region 154 of the roller 151 is comprised of a number of diamond-shaped microscopic areas defined by a plurality of first helices L₅ that are in parallel with each other, and a plurality of second helices L₆ that are also in parallel with each other but arranged so that they are crossed with the first helices L₅. Each of such microscopic areas is provided with a microscopic projection 155.

[0073] In FIG. 15, reference character R denotes a direction parallel to the circumferential direction of the sheet feeding region 154, and reference character W denotes a direction parallel to the center axis of the roller 151. The first helices L₅ are oriented so as to form an angle of 45° with reference to the axial direction W of the roller 151 and spaced from each other by a pitch P₁. The second helices L₆ are oriented so as to form an angle of −45° with reference to the axial direction W of the roller 151 and spaced from each other by the same pitch P₁. Thus, as shown in FIG. 14, the projections 155 are aligned in the axial direction W of the roller 151 so as to be spaced from each other by a pitch P₂. It is to be noted, however, that the directions and the pitches of the first and second helices L₅, L₆ are not limited to those of the embodiments shown in FIG. 15.

[0074] The projection 155 includes a lower portion in the form of a frustum of pyramid, and an upper portion in the form of a pyramid, wherein the bottom surface of the upper portion is smaller than the top surface of the lower portion. The upper portion forms a spike 155A that can be pierced into the sheet S so as to feed the sheet. The top surface of the lower portion can be brought into engagement with the surface of the sheet S as a stopper for limiting the piercing depth D of the spike 155A. The lower portion of the projection 155 has four facets 155C that are opposed to the first or second helices L₅, L₆. Similarly, the upper portion of the projection 155 has four facets 155D that are opposed to the first or second helices L₅, L₆. The first projection 155 forms an angle θ between opposite edges of each facet 155C, 155D. The projection 155 has a height H₁, and the lower portion has a height H₂.

[0075] When the roller body 152 is comprised of a metal material, the sheet feeding surface region 154 can be advantageously formed by a pair of roll forming dies by a roll forming process to be described hereinafter. FIG. 17 shows the arrangement of a roll forming device comprising a first die 160 and a second die 161 for forming the feed roller according to the embodiment of FIG. 14, and FIGS. 18 and 19 are perspective views showing the first die 160 and the second die 161, respectively.

[0076] The first roll forming die 160 each has an outer surface provided with ridges 160A that are arranged at a constant distance over the entire periphery thereof. Neighboring ridges 160A are spaced from each other with a groove 160B therebetween, wherein the groove 160B is of stepped cross-section defined by a trapezoidal portion and a triangular portion. Each ridge 160A is of a trapezoidal cross-section, and has a cutting surface 160C on its top, for forming grooves along the first helices L₅ on the sheet feeding surface region 154. Similarly, the second roll forming die 161 has an outer surface provided with ridges 161A that are arranged at a constant distance over the entire periphery thereof. Neighboring ridges 161A are spaced from each other with a groove 161B therebetween, wherein the groove 161B is of stepped cross-section defined by a trapezoidal portion and a triangular portion. Each ridge 161A is of trapezoidal cross-section, and has a cutting surface 161C on its top, for forming grooves along the second helices L₆ on the sheet feeding surface region 154.

[0077] As seen in exploded views, the angle formed between the ridge 160A and the center axis of the first roll forming die 160 is the same as the angle (e.g., +45°) between the first helices L₅ on the sheet feeding surface region 154 and the center axis of the feed roller 151. Similarly, the angle formed between the ridge 161A and the center axis of the second roll forming die 161 is the same as the angle (e.g., −45°) between the second helices L₆ on the sheet feeding surface region 154 and the center axis of the feed roller 151.

[0078] With such an arrangement of the roll forming device, the facets 155C of the lower portion of the projection 155, which are opposed to the first helices L₅, are formed by the wall surfaces 160E, 161E at the trapezoidal portions of the triangular groves 160B, 161B, the facets 155D of the upper portion of the projection 155, which are opposed to the second helices L₆, are formed by the wall surfaces 160F, 161F at the triangular portions of the groves 160B, 161B, and the stoppers 155B of the projections 155 are formed by the bottom surfaces 160D, 161D of the trapezoidal portions of the grooves 160B, 161B.

[0079] Performance Tests

[0080] In order to confirm functional advantages of the sheet feed roller according to the present invention, performance tests were conducted as follows. First of all, a sheet feed roller 101 shown in FIG. 3(a) was used as Example 1, to measure the sheet feeding distance under various surface pressures of the pinch rollers PR. Another sheet feed roller was used as Control, wherein the projections 106 having the stoppers 106A were replaced by the projections 105 having the spikes 105A, to measure the sheet feeding distance under various surface pressures of the pinch rollers PR. The result of the tests is shown in FIG. 19.

[0081] In the next place, a sheet feed roller similar to that shown in FIG. 3(a) was used as Example 2, to measure the sheet feeding distance under various surface pressures of the pinch rollers PR. In this instance, as shown in FIG. 20, the sheet feeding surface 104X of the feed roller as Example 2 has an arrangement of the projections 105, 106 wherein two projections 106X having stoppers are arranged between two neighboring projections 105X having spikes. The sheet feed roller used as Control was the same as that described above. The result of the tests is shown in FIG. 21.

[0082] It can be seen that FIGS. 19 and 21 are graphs wherein the abscissa indicates the theoretical feeding distance that can be obtained by multiplying the moving angle of the feed roller with the nominal diameter of the feed roller, and the ordinate indicates the deviation of the feeding distance, i.e., the difference between the theoretical feeding distance and the actual feeding distance. It is noted that the surface pressure of the pinch rollers is per 1 mm in the axial direction of the feed roller. The sheets used for the performance tests were coated sheets for ink jet printers.

[0083] In the sheet feed rollers used in the performance tests as Examples 1, 2 and Control, the angle of the first helices L₁ relative to the center axis of the roller is +45°, the angle of the second helices L₂ relative to the center axis of the roller is −45°, the distance between the neighboring helices is 0.35 mm, and the projections with the spikes or stoppers are in the form of pyramid of which the opposite facets form an angle of 50° relative to each other. The dimensions (μm) of the projections are as shown in the following Table. Projections Items Example 1 Example 2 Control Projection Height 75 80 75 with spike Top surface 10 × 10 7 × 7 10 × 10 Projection Height 62 49 — with stopper Top surface  8 × 20 40 × 40 —

[0084] It will be appreciated from FIGS, 19 and 21 that the sheet feed rollers of Examples 1 and 2 according to the present invention exhibit larger average sheet feeding distance due to suppressed fluctuation in sheet feeding, and stable feeding distance due to fluctuations relative to the surface pressure of the pinch rollers.

[0085] With the sheet feed roller according to the present invention, since the projections on the feed surface of the feed roller are comprised of microscopic spikes that can be pierced into the sheet, and stoppers for limiting a piercing depth of the spikes in the sheet, it is always possible to maintain the optimum piercing depth of the spikes by the stoppers even when the hardness of the sheet changes from time to time. In this way, the desired sheet feeding radius or distance can be maintained without causing fluctuations, thereby realizing a highly precise sheet feeding.

[0086] While the present invention has been described above with reference to some preferred embodiments, various modifications or variations may be made without departing from the scope of the invention as defined by the appended claims. 

1. A sheet feed roller having an outer peripheral surface, said outer peripheral surface including at least one feed surface region that extends at least locally in an axial direction of the roller and over an entire circumference of the roller, said feed surface being provided with a plurality of projections, said plurality of projections being comprised of microscopic spikes that can be pierced into the sheet, and stoppers for limiting a piercing depth of the spikes in the sheet.
 2. The sheet feed roller according to claim 1, wherein each of said plurality of projections comprises one of said spike and said stopper.
 3. The sheet feed roller according to claim 1, wherein each of said projections comprising said spike further comprises said stopper.
 4. The sheet feed roller according to any one of claims 1 to 3, wherein each of said projections is arranged in that region of said feed surface, which is defined by first helices extending in parallel with each other on the outer peripheral surface, and second helices extending in parallel with each other on the outer peripheral surface, said second helices crossing said second helices.
 5. The sheet feed roller according to claim 4, wherein at least one projection comprising said spike is arranged alternately with at least one projection comprising said stopper, along said first helices or said second helices.
 6. The sheet feed roller according to any one of claims 1 to 3, wherein each of said projections is arranged in that region of said feed surface, which is defined by generatrices extending along the outer peripheral surface in parallel with an axial direction of the roller, and circumferential lines extending in parallel with each other on the outer peripheral surface, said circumferential lines crossing said generatrices.
 7. The sheet feed roller according to claim 6, wherein at least one projection comprising said spike is arranged alternately with at least one projection comprising said stopper, along said generatrices or said circumferential lines.
 8. The sheet feeder according to any one of claims 1 to 7, wherein said stopper has a flat surface that is substantially at right angles to a radial direction of the roller. 